Power Factors and AC Motors

Who is right? The engineer of course. A motor running unloaded has a poor power factor. Check and see if you are billed a penalty on a KVA basis. If so you may want to increase the PF to 0.90 t0 0.95. You may save more money.

Motor power factor drops sharply as the load decreases. Below about 40% the drop is really steep (from usually about 0.75 it may go as low as below 0.1 on pure no load)

So though you are running the motor at no load, and consuming less active current, you are taking up a very high reactive.

Check up the concept of the production line, why you have to keep on switching On and Off, can you redesign so that the motors can be kept on for longer period?

The starting current, huge, is also primarily reactive, till after a few second it settles at the final value.

And after all these optimisation, put an APFC in line, you may not have an option on this, but the other aspect will improve your power as well as power factor.

Thanks for your input. You have basically confirmed what I was thinking. I can't re-design the production line to run higher loads for longer periods of time because of the process we are running. The motors in question are used to draw a vacuum for our material refill systems. When all of the vessles signal that they are full, a valve is opened so the system doesn't deadhead and open the relief valve. If there is no demand for material we have been shutting the motor off so we aren't needlessly using electricity. Would it be safe to say that a motor that is not running is better for the overall PF than a motor running at no load?

Kevin

Absolutely no doubt about it.

Just be careful about one aspect- Any start stresses the motor - excess current, excess heat, also mechanical stresses on the coil, and a few other things, not recollecting at the moment.

Any way i assume that the start stiop frequency is not too frequent, and there is sufficient time given for the motor to settle down thermally at least (ie motor is cold before you pump in the starting current again).Otherwise you have to have the motor with different duty cycle.

On the average if you follow this aspect, the overall pf will improve by keeping the motors off.

The concept is simple

Motor starting current = 7 x full load - almost wholly reactive for a few seconds + motor running at full load (pf say 0.85) for a few hours

Now just take mathematical average of the pf x time

Else

Motor running at no load - pf = about say 0.15 x time + Full load (pf=0.85) x time

take average

Compare the two pfs. And it will be glaringly visible.

Additional- no rotating machine likes to be run for long duration on low or no load. Bad for its healt (and more for the bearings health) - this we have heard from all the mearing makers. Minimum load the machines to 30% load - else the bearings do not roll, they slip.

Thanks for your help. I hate going to battle without any ammo.

Kevin

hi, one more thing u can try as the no. of motors starts /stop sequence not to be come at same time.Also if possible then motor run with 90% of load so your start /stop time will increased .Also to be arrange to install capacitor bank at MCC end instead of PCC (plant Control Centre). it is very help ful to stabilty of load. can iget what your plant load factor ? if process plant normally we consider as 0.7.

tmp

If you are talking about the power factor, it is .73. We try to sequnce our motors so there is about 5-10 seconds of delay before the next motor starts.

Thanks.

I can't agree more( with Guest). A standing motor ( when not needed) is money in the pocket. When you talk penalties i assume you also work on a maximum demand profile with your power compan, in which case their equipment will measure your demand (use) over a half hour period every half hour. Laymen jargon will have it that it means that if all your motors start up on the half hour measurement moment, the consumption for the next half hour is noted as the current drawn continuously from startup. Yes, the fact that your running current is less than startup, means nothing since the system read your startup as your continuous current. Therfor it makes sense to know your maximum demand cycle, and allow for, or prohibit large units from starting at the same time. I know it is not always possible ( as in my case) but if you setup your start profiles so that all motors start progressivly at maximum intervals you will see a reduction in costs, irrespective of pf equipment installed.( pf will of course aid in reducing your costs anyway) Another idea is to setup your system (if at all possible) so that you can start switching off as you near your dead head, with perhaps a smaller unit doing duty almost continuously, and when full load is required a larger unit comes in to do the job. Remember. if you are running a number of smaller units, say 5 x 15 kw and you replace three of them with a single larger unit (set) of 22kw you will improve your costs. This purely an example, you can play with your own sizes and numbers.

Regards

The best thing you could do is contact an outside company to come in and do a proper energy audit. It could surprise you. At minimum it lays it out to assist the capital planning.

The auditers will come in and, after a preliminary site inspection, setup appropriate data logging. Their reporting is directly geared towards energy saving and lay out the capital money and return on investment... ROI time line. These companies work closely with utility companies to negotiate as much incentive as possible.

The audit I had done last year has been of great value. We immediately replaced or retrofitted every light in the facility. We laid out $275,000. Got back $130,000 from PG&E, our local utility in N. California. Our utility bill immediately dropped $16,000 a month. A 10 month ROI is an absolute no-brainer. I had a couple of old Ingersoll-Rand compressors. A 75hp and 50hp. Gone. New ones have an ROI of about 13 months. New high speed freezer doors are sitting in boxes to be installed over the Xmas New year week.

We identified a few spots where pf caps went on at the motors but really wasn't all that significant. What was a good help was negotiating a different Kvar penalty rate based on the overall effort at energy reduction and efficiency.

So.. best bet would look around your local area, talk to a few people and get an audit completed so you know specifically what savings are to be had and where the best items to correct are.

I think you are right. An audit should be performed. I believe the ROI would be less than 5 years. We've already done the lights. We switched from metal halide bulbs to a more efficient flourescent bulb. That helped a ton. We've also done things such as closing overhead doors in areas like the warehouse where there aren't people working all of the time so we don't have to heat them. Locking thermostats also helps. Before there were locks on the thermostats, there would be areas of the plant where it would feel like summer during the winter.

At any rate, I was presented with the request to stop turning the motors off when the machine didn't require it's operation because it was supposed to increase our power factor.

Thanks for your advice.

last night while on the way I had another thought. The idle running motor is still running the compressor - it is not decoupled - so it is actually on no load, only the compressor vacuum is now by-passed to air ie it is on closed loop - it draws the air from atmosphere and discharges back.to it.

If it is like this , the unnecessarily running motor however will be a wastage of active power rather than reactive power.

But switched off motor is good in all the ways.

Check the pf of the other loads. Now I have doubt about them, all these lighting and other loads are likely to be inductive rather than resistive. Check their pf.

Dear Friends,

I think one thing what we have over looked is if your motors are synchronous motors you can generate reactive power when it is running on no load which leads to overhaul imrovement of power factor.In the case of induction motors it is other way.The ideal option for you is go for VFD or Power Boss.

I've checked entire comments on the thread.

The best solution is installing a Power Factor Corrector with a suitable capacitor bank, controlled with a reactive power relay. It is totally automatic and if correctly set, then no need to care whether motor is starting or running at no load.

One other point, motors running with lower loads, or running idle, are creating reactive power. This is very correct.

But as far as I understand this is your working regime and you can not change the situation. Of course Power Factor corrector will correct this if adequately designed.

The essential point I would like to negotiate here is the situation during motor starts.

I do not agree on the matter, the motors are creating much reactive power during startup period. Back EMF at the windings of the motor is at the minimum value during startup.

I would be pleased to here different ideas

http://lejpt.academicdirect.org/A06/17_28.htm

The active and reactive power absorbed by the induction motor in transient starting are depicted in fig. 9. The measurement system was tested with induction motor without load, it is clear that the active power absorbed by the motor is negligible but there is a little amount of reactive power necessary to create the three phase rotating magnetic field in the stator of the induction motor.

Here at clause 3.2 example

Example. U = 6.6 kV, IS = 68 A and cosΦ = 0.85. Motor start current is 7.8 x IS = 530.4 A and cosΦ = 0.15.

The concept is simple (but had to give some links )- At start, the magnetic field is set up then only the power transfer takes place. Though the torque is a bit high (though not full load) - but the ω is negligible, so active power is negligible. (τω) still we are drawing 7 times current at the rated voltage.

Thank you for the links.

I will check them As soon as possible

Regards

I checked both links. (As carefully as I can)

Power factor for a blocked motor (Or at startup) is not clearly mentioned in neither links.

( To avoid any misunderstanding, I think it is good to mention here that :

It is obvious there is a high current available. (because it is a startup step)

But this high current does not mean the high time lag (Reactive power) is available.

Kindest Regards

A transient starting test of induction motor was carried at the laboratory with the proposed system.

http://www.rose-hulman.edu/class/ee/HTML/ECE470/PDFs/lecture_12_outline.pdf

http://www.lmpforum.com/inforum/Starting-Philosophy-Induc-t1932.html

http://ecmweb.com/mag/electric_power_factor_correction/

http://ecee.colorado.edu/~ecen4517/materials/InductionMotor.pdf (page 6 last paragraph)

However, the starting torque of the low-resistance squirrel-cage induction motor is relatively low as seen in Fig. 3. This can be explained in a practical manner by referring to the equivalent circuit and realizing that since the slip is 1 at start, the rotor branch impedance is simply RR + jwLR and the power factor is low. This low rotor power factor is responsible for the low starting torque.

Though really so many links were not required. You have High Current at start, Full Voltage, so High KVA, Low Torque, Low Speed (Hence Low KW).

High KVA, Low KW = Low pf.

Does anyone know if this problem is being addressed by the smart grid concept?

To reduce yr monthly consumption,u need to avoid switching ON loads that are not so immediately needed.But what u need most is to correct the poor pf of O.73 to about O.95.but yr boss says he needs O.88. take the following method:

U need to improve yr pf from 73% to 88%(ie from O.73 to O.88).Get yr kw,voltage,frequency f and yr kva.

Take the following method using the parameters above.

kva=kw/O.73; improved kva=kw/O.88; improved kvar=√(improved kva²-kw²)

The kvar value calculated is what u need to correct yr pf from 73% to 88% and the capacitor(s) should be connected in parallel to yr load.U may want to calculate yr capacitive reactance as well as yr capacitance value in μf,using:

X=E²/Q ;(Q=KVAR, X=Capacitive reactance in Ω, E =Potential-voltage); Xc=1/2ΠfC; C=1/2ΠfXc

By the time u finish and plot yr power triange,u will find out that yr new reduced kva line will almost come close or meet with the kw line with a smaller kvar line linking both.The kva will have a value almost close to the kw value.

Yr pf may have been improved or corrected to 88% using the appropriate capacitor(s) as calculated in parallel with yr load.

Patrick Whowha

P.F. is a part of power quality enhancement

Today we use multiple frequency systems - as soft start and U.P.S. these are Infected the grid -

If you give me more info on your system i maybe help you as consultant on power quality -

Joshua Gabriel SAADA

Alternative Power Systems
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There are two ways to operate your system:(A) utilize a large volume vacuum accumulator with a differential vac-flow switch tied into either a VFD sequenced drive to ramp up and down on demand ....base the accumulator volume on all points of use being used simultnaneously for 30 seconds. Then have the VFD ramp up the motor or motors....hope you have a large ring main in your plant and not a number of pumps with motors.

If you are using the vacuum just for material handling "Roots blowers" can be used instead of true vacuum pumps....more amenable to varying speed if you have shut off valves at each point when not being used....place vacuum flow sensor tied into VFD control panel along with a small PLC with variable logic taking info from each use point shut off valve (use a limit switch to send signal to PLC)...willdefinitely cut your energy costs and spikes (especially if you pay based on demand)

(B)You can also use a Soft-start" motor starter that also ramps up...stops the Amp spike associated with a DOL system. (same as A in energy reduction but not really variable demand capable...still on/off)

You really need to look at your whole system via an audit and its REAL use not just the motors.

Ring main from vacuum accumulator, end use shut off valves with sensors, flow and vac sensors tied into a PLC program and a suitably sized VFD drive and motor needed to maintain the Acuumulator use and appropriately sized blower to get the vacuum needed (may not need a water-ring type vacuum pump-closed loops are avaialble to conserve water useage).

Your manager may have to make some changes but the long term savings will be quickly realized...but remember to look at three to five years down the road to ensure future needed/expected capacity...paid with todays dollars NOW, may not be fully utilized but is there ready and no waiting to be purchased/installed etc or loss or disruption of production in the future.

Geoff NH